If you’ve already heard that dinosaurs are extinct — and the betting is that you have — you’ve probably also heard the reason: an asteroid smashed into Mexico’s Yucatán Peninsula some 65 million years ago, blasting enough dust into the atmosphere to block the sunlight and dim the skies. These days that’s Intro to Dinos stuff, but prior to 1980, the thinking was very different. Dinosaurs endured a long, slow decline, the theory went, ceding their rule of the planet only gradually and grudgingly. The old theory is nearly as extinct as the beasts themselves — or at least it was until last week, when a new study pumped a little life back into it. About 200,000 years before the asteroid hit, a separate extinction was already under way, wiping out numerous species of clams and snails on the ocean floor. And it was terrestrial volcanoes, not a rock from space, that were to blame.

The new findings, published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, are the product of three years of summer visits by researchers to Seymour Island, on the Antarctic Peninsula. Polar regions make research hard — one year the ice was so thick that the scientists’ boat couldn’t reach the shore — but when the weather cooperates, the sites offer fossil hunters distinct advantages. One is that, like today, the ancient poles were more susceptible to climate change. Ice then was the same as ice now, which is highly reflective. As it melts, it gives way to darker ocean water, which absorbs more light and heat, exacerbating warming in an accelerating loop. In this case, scientists wanted to determine temperatures in the polar ocean from 65 million years ago, so they gathered more than 100 ancient, 2-in.-wide (5 cm) shells from subterranean samples, then analyzed the minerals and calcium carbonate that had formed the shells.

Those ancient artifacts revealed a lot. Calcium carbonate — an organic stew of calcium, carbon and oxygen — is nature’s answer to a stuck thermostat, with the environmental conditions when the material formed forever stamped into its chemical makeup. The oxygen portion has two variants: oxygen-16 and oxygen-18. If there’s more oxygen-16, seawater temperatures were higher when the shell formed — and that’s exactly what the scientists discovered, in this case an estimated 9ºF (5ºC) temperature jump from typical seawater temperatures. By matching that information with the strata layers where those oxygen-16-rich shells were found, geologists identified three periods of ocean warming: about 2 million years before the great dinosaur extinction, then 250,000 years before and again about 200,000 years after.

All three of those eras coincide nicely with volcanic events in India’s Deccan Plateau, which may have belched enough carbon dioxide and other greenhouse gases to boost planetary temperatures and harm ocean dwellers. “If this warming is severe enough to be impacting marine life, it’s likely impacting terrestrial life too,” says Tom Tobin, a doctoral student at the University of Washington and the report’s lead author. “So it’s not unreasonable to think that the environmental stress from the volcanism contributed to the strength of the asteroid extinction. One of [the events] is synchronous with the marine extinction that we found 200,000 years before the asteroid impact.”

Such relatively specific dates underscore another reason the scientists chose their Antarctic research site: it offers unusually thick, 3,200-ft.-deep (1,000 m) sediment, and the deeper the sediment is, the more layers of geologic time have been preserved. This region apparently built up more than most because of repeated sediment deposits during uplift of the mountain range that makes up the Antarctic Peninsula. In fact, the Seymour Island site has so many sediment layers that scientists could whittle the time intervals down to hundred-thousand-year scales — a minuscule window compared with the 10-million-year degree of accuracy most sites provide.

Even this geologic precision, however, wasn’t enough to seal the deal, at least not to an acceptable scientific certainty. So the scientists used an imaginative method, matching changes in Earth’s magnetic field to different layers in the strata. Though most people assume magnetic north is a constant, it isn’t. Every 500,000 years or so — and in random periods ranging from as little as 100,000 years to 30 million years — the magnetic north pole flips to the south pole. When it does, magnetic particles in slow-building sediments orient themselves accordingly, then become locked in place as further sediment builds.

With this in mind, the researchers drilled core samples and compared the particles’ orientation with the known history of magnetic-field changes. That provided confirmation that the earlier oceanic extinctions occurred within the range of Deccan volcanism. “Once we compared the warming record to the timing of the volcanism,” says Tobin, “we realized, Hey, these things seem to line up pretty well. This is pretty cool. It brings more complexity and nuance to the picture we already had.”

Intrigued by the possibility of further reconciling extinction theories, the researchers are searching for a second site with the rare, deep sediment layers necessary for a close-up look at the late-Cretaceous period. For the moment, it seems that both volcanoes and the infamous asteroid may have contributed to the dinosaurs’ demise — a tidy addendum to Dino 101.